In recent years, a number of medical devices have been designed which are adapted for compression into a small size to facilitate introduction into a vascular passageway and which are subsequently expandable into contact with the walls of the passageway. These devices, among others, include blood clot filters which expand and are held in position by engagement with the inner wall of a vein, such as the vena cava. Such filters include structure to anchor the filter in place within the vena cava, such as elongate diverging anchor members with hooked ends that penetrate the vessel wall and positively prevent longitudinal migration of the filter within the vessel.
A number of conditions and medical procedures subject the patient to a short term risk of pulmonary embolism which can be alleviated by a filter implant. In such cases, the filter catches and retains emboli to prevent them from reaching the lungs or the brain. A number of configurations of blood filters are known. An example of such a filter is disclosed in U.S. Pat. No. 6,258,026 and illustrated in FIGS. 16 and 17.
Typical previously known blood filters include a number of locator members 20 and anchor members 30. The locator members 20 and anchor members 30 may be offset one from the other about the longitudinal axis of the filter 1, as shown in FIG. 17. Hooks 40 positioned on the distal ends of the anchor members 30 engage the blood vessel wall to prevent longitudinal movement within a blood vessel. When in place in a blood vessel 6, anchor members 30 form a first filtering zone 10 and locator members 20 form a second filter zone 11, as shown in FIG. 18.
First filtering zone 10, which is defined by anchor members 30, receives blood flow 8 before blood reaches the remainder of the filter 1. So emboli 5 tend to be captured preferentially in first filtering zone 10, as illustrated in FIG. 18. This result has disadvantages when a significant mass of emboli 5 are retained. For one, under the pressure of blood flow 8, the emboli 5 will press with a force F0 against anchor member 30 which results in a radial force component F2. Acting as a lever, anchor member 30 translates the radial force component F2 into greater stress 100 applied to the blood vessel walls 6 by the hooks 40. Over time, the stress 100 applied to the vessel walls 6 may lead to vascular injury or disease. In addition, since the anchor members 30 must engage the vessel wall to hold the filter 1 in place, the first filtering zone 10 is susceptible to complete filling such that emboli 5 span the entire cross section of the blood vessel, as illustrated in FIG. 19. If this happens, in addition to increasing the stress 100 applied to the blood vessel wall 6, the captured emboli will completely block blood flow through the vessel.
Accordingly, there is a need for a blood filter which will not increase stress applied to blood vessel walls as emboli are captured and will not completely block blood flow if filled with emboli.